The invention relates to a piezoelectric motor, particular for use as miniature electrical motors adapted for continuous or stepped rotational movements. The motor according to the invention can be used in automation systems, in robot techniques, in machine tools to precisely position cutters, in vehicles as screen wiper motors and pane lifter motors and as a drive means for seat adjustments. It can also be used in inertialess drive means for TV aerials and other devices which require a great moment of rotation at a comparatively low speed of rotation.
Piezoelectric motors are known in which the electrical energy is converted into a rotational motion of a rotor by means of piezoelectric oscillators which comprise two resonators of two different standing waves as disclosed in the U.S. Pat. Specification No. 4,019,073, for example. With this kind of motor it is difficult to match the two different types of standing waves over a wide range of temperature as well as under mechanical stress. Said disadvantage is not inherent in piezoelectric motors which are based on the principle of generating acoustic travelling waves by means of uniform standing waves (refer to Axel Froschler, Analyse eines Piezo-Wanderwellenmotors, Dissertation, Stuttgart, 1992, pg. 48 ff.). These motors are disadvantageous due to the use of acoustic waves which require the piezoelement which generates the travelling wave being embodied as a thin ring cemented by an elastic organic adhesive to a metallic waveguide.
Due to the use of elastic waves and due to the small volume of the piezoelement, compared to the entire volume of the waveguide, the efficiency factor of the electromechanic energy conversion is low which requires a considerable increase of the excitation voltage for the motor. The connection between the piezoelement and the metallic waveguide by means of an organic adhesive restricts the maximum mechanical motor output, reduces its efficiency factor and the reliability. The piezoelement of a motor employing such waves exhibits differently oriented polarizations which renders the manufacture difficult and the motor expensive so that piezoelectric travelliing wave motors cannot compete with the cheap electromagnetic motors.